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X-raying the beating heart of a newborn star

03 July 2012

An international team of scientists has used the world's most powerful X-ray observatories - including ESA's XMM-Newton orbiter - to probe the dusty surroundings of a newborn star and discover some of its innermost secrets. These findings shed new light on one of the most fundamental processes in the Universe, the creation of stars.

Our Galaxy contains numerous clouds of gas and dust - stellar nurseries where stars are born as the result of gravitational collapse and gradually grow in size until hydrogen fusion begins, enabling them to blaze forth in all their glory. Although the basic outline of the story is fairly well understood, there are still many questions that need to be answered.

It is generally believed that infant stars – known as 'protostars' – grow as the result of mass accretion. Large amounts of matter fall onto them from the innermost part of a surrounding disc, which is created by the gravitational collapse of a molecular cloud.

Fast-moving jets of material have been observed flowing outward from many protostars, suggesting the influence of powerful magnetic fields and highly energetic processes in the innermost regions of protostellar discs. However, the presence of the cocoon of gas and dust makes it extremely difficult to discover what is happening to the fledgling star in the centre.

In order to learn more about the growth of protostars, the team decided to use data from three orbiting X-ray observatories - XMM-Newton, Chandra and Suzaku - to study a young, low-mass star, known as V1647 Ori, which is located at the apex of McNeil's Nebula.

Soaring temperatures and X-ray spots

Writing in The Astrophysical Journal, an international team of scientists has now re-examined X-ray data obtained during two optical outbursts that are associated with mass accretion on V1647 Ori – one that occurred during the period 2003-2006 and another that has been under way since 2008.

Previous studies had shown that the protostar's X-ray output increased 100 times when the optical outbursts occurred, whilst the temperature of the plasma soared to about 50 million Kelvin. However, the cause of these soaring temperatures was unclear. Acceleration of this material due to the influence of gravity alone is insufficient to raise the temperature of the plasma above a few million degrees.

"In some ways it's like a huge waterfall, cascading down under gravity," said Kenji Hamaguchi from NASA's Goddard Space Flight Center, lead author of the paper. "We found that the 50 million degree plasma produced by mass accretion activity is located at the bottom of the accretion flow.

"In order to explain the sizzling temperature, the plasma stream must be hitting the star's surface at a speed of around 2000 km/s. The most likely explanation is that magnetic reconnection – a sudden reconfiguration of the magnetic field lines close to the young star - is accelerating the material."

The team also identified a regular, short period, variation in the protostar's powerful X-ray emissions.

"During the two outbursts, we identified strong similarities in 11 separate X-ray light curve observations of V1647 Ori, obtained with three different space observatories," said Hamaguchi.

The light curve with the longest duration, obtained with XMM-Newton in 2005, shows that the X-ray flux stays constant for about 5½ hours, rises by a factor of 5 over the next four hours, remains at an elevated level for more than 8 hours and then falls gradually to the original flux level.

"Subsequent observations with Suzaku enabled us to identify a similar light curve, indicating a periodic variation in the X-ray emission that lasts about one day," said Nicolas Grosso, a CNRS researcher at Strasbourg Astronomical Observatory in France, and a co-author of the paper.

"Since V1647 Ori has an estimated mass of 0.8 solar masses and a radius about five times larger than the Sun, this periodicity indicates that the star is rotating so quickly that it is close to breaking apart."

What is causing the periodic changes in X-ray output? According to the team, the most likely interpretation is that localised 'hot spot' regions of X-ray plasma are moving in and out of our line of sight as the star rotates. Rises and falls in the light curves would then correspond to appearances and disappearances of X-ray bright spots.

"The phases of low and high flux in the light curve cannot be reproduced by a single spot," said Hamaguchi. "We, therefore, assume two spots with identical shapes, located on opposite sides of the star."

X-ray lightcurve of V1647 Ori. Credit: Hamaguchi et al.

Modelling of the X-ray light curve indicates that the extremely hot plasma resides in large, pancake-shaped magnetic footprints, where the material from the disc is colliding with the surface of the newborn star.

The sustained X-ray periodicity of V1647 Ori demonstrates that such protostellar accretion can be stable over timescales of years. The duration of the rises and falls in the flux suggests that the spots cover a large area of the protostar's surface.

"The footprint of the infall is comparable to the size of the Sun's disc" said Hamaguchi. "Our calculations suggest that the bright spot is about five times more luminous than the faint spot. The brighter spot is located at a stellar latitude of about 49 degrees, whereas the star's inclination - the tilt of its polar axis toward our line of sight - is about 68 degrees. This alignment enables us to detect the accretion footprint."

"How stars are created is one of the fundamental questions in modern astrophysics, so studies such as this, which reveal the physical processes at work, are extremely important," said Norbert Schartel, ESA's Project Scientist for XMM-Newton.

Notes for editors

The study described in this article was carried out using data from ESA's XMM-Newton, NASA's Chandra and the Japan/US Suzaku observatories.

V1647 Ori, a young stellar object located at the apex of McNeil's Nebula, has erupted in optical outbursts on two occasions in recent times: once during 2003-2006 and again in 2008 - the latter outburst is still ongoing.

McNeil's Nebula, which lies in the constellation Orion, about 1300 light years from Earth, was discovered by amateur astronomer Jay McNeil in January 2004. The nebula became visible after a sudden brightening of the infant star in October-November 2003 illuminated the material around it. This outburst was caused by a dramatic increase in the rate of mass accretion from the surrounding disc.